In liquid-jet printers, more particularly matrix printers for large printing with individually printed characters or symbols which, depending on a control unit, are continually changed, very high demands are made on the grouping of those valves which are located in each individual channel and which switch between open and closed positions in order to eject those liquid droplets under pressure which, according to a pattern determined by the control unit, are ejected onto an object which, for instance, moves past the print head. With unsatisfactory grouping of the valves, a risk of leakage arises such that the print liquid collects in the outlet openings of the print head also during periods between each print moment, for example, during changing from one object to another. Thus there is a risk that the print liquid thickens and dries due to its exposure to air, with the subsequent risk of blocking the outlet openings in the print head.
The above problem can be partially solved by manufacturing the valves with high quality seals, though in practice a certain amount of wear nevertheless occurs and thus periods of reduced sealing and risk of leakage arise. Even with high quality seals, a small quantity of print liquid remains in the outlet openings, with the risk of blockage if the intervals between the print moments are too long.